The accumulation of fireside deposits on the internal heating surfaces of boilers drastically reduces their thermal conductivity and efficiency and, if not removed, requires periodic shutdowns of the boiler for manual cleaning. The principal means for removing fireside deposit accumulation in boilers is a cleaning device known as a sootblower. A conventional sootblower typically consist of a lance tube having a plurality of nozzles which direct jets of a compressible cleaning agent under pressure, such as steam, gas or vapor, sidewise from the lance against the internal surfaces of the boiler. The cleaning effectiveness of a sootblower depends to a great degree on the nozzle design which controls the mass flow, exit speed and the jet decay characteristics of the exiting jets.
The sootblower nozzle design most commonly used today is based on the de Laval design comprising convergent and conical divergent flow sections which form a venturi. The pressure of the cleaning agent decreases as it passes through the convergent segment of the nozzle, attaining the local speed of sound at the throat of the nozzle. The pressure of the cleaning agent then decreases further through the conical expansion section, expanding and accelerating from the nozzle throat to the nozzle exit and thereby typically exceeding the speed of sound as the cleaning agent exits. The pressure drop over the expansion section is controlled by the designed geometry of that section, primarily the divergence angle and length. Conventional belief is that the optimum divergence angle is about 15.degree. or less so as to prevent the attendant generation of turbulence.
The cleaning potential of the jet emitted from a nozzle is commonly measured in terms of the jet's Peak Impact Pressure (PIP). The maximum PIP is delivered by nozzles where the pressure of the cleaning agent jet exiting the nozzle jet equals the ambient pressure surrounding the lance tube, thereby resulting in a "fully expanded" jet. Nozzles which allow the pressure of the exit jet to be greater than the ambient pressure result in an "under expanded" jet. In the case of under expanded jets, the pressure of the exiting jet is higher than the ambient pressure so the exiting jet must finish expanding outside the nozzle causing a series of expansion and contraction waves called "shock waves." These "shock waves" convert a substantial part of the kinetic energy of the jet stream into internal energy, thereby markedly reducing the PIP.
A "full expansion" nozzle is achieved by designing the nozzle with a specific ratio between the area of the nozzle's exit to the area of the nozzles's throat. The ratio is determined by the particular nozzle inlet pressure. In practice, this means the length of the expansion segment of the nozzle, L.sub.n, needs to be extended to allow for the full expansion and the corresponding drop in pressure of the cleaning agent down to the ambient pressure at the nozzle's exit. However, the size of the sootblower lance tubes as well as the openings in the boiler wall through which the lance tube is inserted limit the elongation of conventional nozzles to achieve full expansion. This is shown in Table I where the prior art full expansion nozzle requires a nozzle length of approximately 3.5 to 5.0 inches. However, the inside diameter of the lance tube to which these nozzles are attached is only about 3.0 inches, restricting conventional nozzle lengths to approximately 1.63 inches. Furthermore, the sleeve diameter of the opening in the boiler wall through which the lance tube is inserted dramatically restricts the projection of the nozzle outside the lance tube. Table I below gives a comparison of the nozzle lengths of conventional nozzles which are under expanded and the same nozzle made full expansion.
TABLE I ______________________________________ Conventional Under Expanded Full Throat Flow Nozzle Expansion Nominal Size Area Rate* Length Nozzle Length (in.) (in..sup.2) (lbs/sec.) (in.) (in.) ______________________________________ 7/8 0.601 2.24 1.63 3.45 1 0.785 2.93 1.63 3.86 1 1/8 0.994 3.71 1.63 4.95 ______________________________________ *For 300 psi inlet pressure and 600.degree. F. superheated stewn.
Consequently, the shorter under expanded nozzles are used in conventional sootblowers. These circumstances are most apparent with the so called long retractable sootblowers, such as the one disclosed in European Patent No. 159,128. The sootblower of the '128 patent uses a lance tube typically having a plurality of under expanded nozzles at its working end which are generally positioned opposite to each other, with aligned center axes or slightly staggered center axes in order to offset the jet reaction forces, as seen in FIG. 2 of the '128 patent.
A nozzle designed to emulate the characteristics of a full expansion nozzle while having dimensions allowing it to be incorporated into a sootblower lance tube is disclosed in U.S. Pat. No. 5,271,356 to Kling et al. The nozzle device taught in the '356 patent utilizes a plug mounted to the back wall of the lance tube or supported by a radially extending support vane, as seen in FIGS. 4 and 5 of the '356 patent. Inherent with such a design is the workmanship involved in fabricating and mounting the plug and nozzle outer shell. Moreover, the plug must remain concentric in respect to the nozzle outer shell or the nozzle performance is diminished.